The heat produced by a machine tool during operation thereof over a long period of time and the changes in the environmental temperature will lead to thermal deformation of the machine tool as a result of thermal effect. In this case, the relative position between the cutter and the workpiece on the machine tool will change to cause errors in the dimensions and shape of the workpiece being machined and accordingly, lowered machining accuracy. Generally, 40%˜70% of the machining errors are attributable to the thermal deformation of the machine tool. Therefore, the thermal behavior of the machine tool is considered an important index for the machining accuracy and stability thereof. A repeatedly and stably occurred thermal behavior indicates the machine tool can maintain good machining quality over a long duration. On the other hand, a machine tool having extremely variable thermal behavior will have trouble ensuring its machining quality. Therefore, many manufacturers deem their solutions for heat-induced errors, such as the thereto-friendly technology and the heat source cooling and suppressing technology, a technical symbol for high machining accuracy and high quality stability.
The method used by domestic machine tool-related manufacturers to solve the problem of thermal deformation of machine tool is to develop computerized numeric control (CNC) system-based thermal deformation control technology. This thermal deformation control technology includes (1) the mounting of temperature sensors on the machine body at positions with relatively significant temperature variation, which can be measured with an infrared thermography, to capture temperature changes of the machine tool; (2) the setup of a three-dimensional measuring system for measuring and recording the deformation of the machine tool caused by temperature rise; (3) the use of temperature data and thermal deformation data to construct a machine tool thermal deformation model; and (4) the testing of the machine tool thermal deformation model and temperature rise compensation.
According to the CNC system-based thermal deformation control technology, the temperature variation amount of the machine tool is measured and calculated using the built-in software of the machine tool thermal deformation model to derive the real-time thermal deformation amount of the machine tool, which is then used as a thermal compensation and calibration displacement amount for the machining spindle. In the development of the thermal deformation model software, since the sampling range of the machine tool heat source and the environmental temperature is limited, a big change in the heating feature of the built-in motor of the machine tool or a big change in the environmental temperature will cause errors in the initially set calibration software calculation when the machine tool has been used over a long period of time, so that the working accuracy and the machining stability of the machine tool will reduce.
Many methods have been adopted by foreign manufacturers to reduce the thermal deformation amount of the machine tool. These methods include (1) the designing of a machine body structure with thermal symmetry and thermal balance, so that the occurrence of the machine tool thermal deformation induced errors has symmetry and is controllable; (2) the adoption of thermo-friendly machine body to effectively reduce the thermal deform amount of the machine body of the machine tool; (3) the adoption of multi-passage zero-heat source cooling technology to effectively reduce the temperature variation amount of the heat source; and (4) the measurement of hot points on the machine body and the compensation for thermal deformation. The above-mentioned technologies have their respective functions and must be fully implemented to meet the preset specifications, and they also involve very high complexity, difficulty and manufacturing cost.
Therefore, it is desirable to develop a measurement, calibration and compensation system and method for machine tool, so that the thermal expansion amounts at all axes of a machine tool can be directly precisely measured through a simplified and low-cost way with good thermal deformation calibration accuracy, and the absolute positioning coordinates of all axes of the machine tool can be calibrated in real time to avoid reduced positioning accuracy due to the thermal expansion of the multi-axis machine tool.